An example of such an electro-acoustic transducer is known from US 2006/0157841, which discloses a silicon condenser microphone package including a transducer unit, a substrate, and a cover. The transducer unit is attached to an upper surface of the substrate and overlaps at least a portion of a recess wherein a back volume of the transducer unit is formed between the transducer unit and the substrate. The cover is placed over the transducer unit and either the cover or the substrate includes an aperture.
Generally, the membrane of an electro-acoustic transducer (or a pressure sensor) just senses the difference of the pressure in front of the membrane and behind the membrane. For a microphone this means that sound waves vary the pressure in front of the membrane whereas the pressure behind the membrane is held substantially constant. This is done by means of a back volume, which for high sensitivity of the transducer shall not be too small. Roughly speaking, the back volume depends on the compliance of the membrane for a given sensitivity. Because a membrane cannot be arbitrarily compliant, the back volume has to have a certain size. However, a big back volume is a contradiction to the ever decreasing size of electronic devices.
According to prior art, a back volume is either formed in the die of the MEMS sensor and/or in the substrate, to which the MEMS sensor is attached, and/or in the PCB (printed circuit board) of the device, to which the transducer is attached (e.g. the PCB of a mobile phone). For this reason the measures to obtain a back volume with a sufficient size are relatively extensive. Hence, it is an object of the invention, to propose an electro-acoustic transducer which overcomes these drawbacks.